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re: winter humidification wastes energy 11 feb 2005 >ok nick, care to elaborate on what your theory is? sure. here's the email i sent to lennox... sent: monday, january 17, 2005 8:58 am subject: attn: president/legal--winter humidification wastes energy gentlemen, i suspect that winter humidification wastes vs saves heating energy, and the savings claim is an energy myth. people tend to forget that evaporating water takes heat energy, and that heat energy has to come from somewhere, even if something like a humidifier belt motor uses little energy by itself. the heat saved by turning a thermostat down appears to be far less than the extra heat used to evaporate water, in all but extremely tight houses with little insulation, eg submarines. http://lennox.com/pdfs/brochures/lennox%20wb2-wp2%20humidifiers.pdf claims that 69 f at 35% rh and 72 f at 19% rh are equally comfortable, but the basic program in the new ashrae 55-2004 comfort standard predicts that 69 f and 35% rh and 69.7 at 19% rh are equally comfortable (pmv = -0.537, see below.) if a 2400 ft^2 tight house has 0.5 ach and say, 400 btu/h-f of conductance, turning the thermostat down from 69.7 to 69 saves (69.7-69)400 = 280 btu/h. air at 69 f and 100% rh has humidity ratio w = 0.015832 pounds of water per pound of dry air, so 19% air has wl = 0.00301, and 39% air has wh = 0.00617. raising 69 f air from 19 to 39% requires evaporating wh-wl = 0.00316 pounds of water per pound of dry air. dry air weighs about 0.075 lb per cubic foot. with 0.5x2400x8/60 = 160 cfm or 9600 ft^3/h or 720 pounds per hour of air leakage, raising the indoor rh from 19 to 39% requires evaporating 720x0.00316 = 2.275 pounds of water per hour, which requires about 2275 btu/h of heat energy, so it looks like humidifying this fairly airtight house wastes 2275/280 = 8 times more energy than it "saves." and many us houses are less airtight, so humidification would waste more energy. please modify your energy-savings claim. thank you. nick pine 10 screen 9:key off 20 clo=1'clothing insulation (clo) 30 met=1.1'metabolic rate (met) 40 wme=0'external work (met) 50 data 69,35,69.74,19 60 for case=1 to 2 70 read tc,rc 80 ta=(tc-32)/1.8'air temp (c) 90 tr=ta'mean radiant temp (c) 100 vel=.1'air velocity 110 rh=rc'relative humidity (%) 120 pa=0'water vapor pressure 130 def fnps(t)=exp(16.6536-4030.183/(ta+235))'sat vapor pressure, kpa 140 if pa=0 then pa=rh*10*fnps(ta)'water vapor pressure, pa 150 icl=.155*clo'clothing resistance (m^2k/w) 160 m=met*58.15'metabolic rate (w/m^2) 170 w=wme*58.15'external work in (w/m^2) 180 mw=m-w'internal heat production 190 if icl<.078 then fcl=1+1.29*icl else fcl=1.05+.645*icl'clothing factor 200 hcf=12.1*sqr(vel)'forced convection conductance 210 taa=ta+273'air temp (k) 220 tra=tr+273'mean radiant temp (k) 230 tcla=taa+(35.5-ta)/(3.5*(6.45*icl+.1))'est clothing temp 240 p1=icl*fcl:p2=p1*3.96:p3=p1*100:p4=p1*taa'intermediate values 250 p5=308.7-.028*mw+p2*(tra/100)^4 260 xn=tcla/100 270 xf=xn 280 eps=.00015'stop iteration when met 290 xf=(xf+xn)/2'natural convection conductance 300 hcn=2.38*abs(100*xf-taa)^.25 310 if hcf>hcn then hc=hcf else hc=hcn 320 xn=(p5+p4*hc-p2*xf^4)/(100+p3*hc) 330 if abs(xn-xf)>eps goto 290 340 tcl=100*xn-273'clothing surface temp (c) 350 hl1=.00305*(5733-6.99*mw-pa)'heat loss diff through skin 360 if mw>58.15 then hl2=.42*(mw-58.15) else hl2=0'heat loss by sweating 370 hl3=.000017*m*(5867-pa)'latent respiration heat loss 380 hl4=.0014*m*(34-ta)'dry respiration heat loss 390 hl5=3.96*fcl*(xn^4-(tra/100)^4)'heat loss by radiation 400 hl6=fcl*hc*(tcl-ta)'heat loss by convection 410 ts=.303*exp(-.036*m)+.028'thermal sensation transfer coefficient 420 pmv=ts*(mw-hl1-hl2-hl3-hl4-hl5-hl6)'predicted mean vote 430 ppd=100-95*exp(-.03353*pmv^4-.2179*pmv^2)'predicted % dissatisfied 440 print tc,rc,pmv 450 next case 69 35 -.5376486 69.74 19 -.5372599 engineering vp mark hogan said lennox was embarrassed by all this and he didn't know where their numbers had come from, and he thanked me for bringing this to their attention and said they are changing their printed brochures and aprilaire web site energy-savings claim. this reminds me of david and goliath :-) nick |